1. Field of the Invention
The present invention relates to a mold apparatus for forming an article under the influence of a magnetic field and, more particularly, to a mold apparatus suitable for use in a process for forming a synthetic resin magnet in which a mixture of a synthetic resin and a ferromagnetic powder is injection-molded in a magnetic field to form a ring-shaped synthetic resin magnet magnetized uniformly in the radial direction.
2. Description of the Prior Art
Ring-shaped magnets are often used in dynamic electric machines or devices such as D.C. motors, generators, hysteresis motors, step motors and so forth. Hitherto, a sintered magnet of barium ferrite or strontium ferrite has been used in said fields. Such known magnet materials, however, exhibit a thermal shrinkage of about 20% in volume when cooled after the sintering, resulting in an inferior dimensional precision and, in the worst case, cracking is caused in the product. The ring-shaped magnet produced from such materials requires a polishing of the surface of the sintered body to achieve the desired dimensional precision, and the polishing constitutes one of the factors raising the production cost of the magnet. It is also known to produce the ring-shaped magnet by a method other than sintering, e.g. by injection- or extrusion-molding a mixture of a ferromagnetic powder and a synthetic resin or a rubber. The use of a resin as the matrix material provides a magnet having a higher dimensional precision and facilitates the production of articles having complicated shapes as compared with the production of a magnet by sintering. However, the plastic magnet contains a synthetic resin, which is non-magnetic, as the matrix, and hence, exhibits an inferior magnetic force to that of the sintered magnet. Therefore, various attempts have been made to improve the magnetic force by imparting an anisotropy to the magnet.